1. Field of the Invention
This invention relates to a radio telephone system switching control capability of control channels such as a land mobile telephone system of cellular type.
2. Description of the Prior Art
There is known a land mobile telephone system wherein calls are placed from a mobile station such as an automobile via a base station which conducts radio relays. This is quite different from ordinary telephone sets which are fixed in a public telephone box or at an office and wired to an exchange station. As the land mobile telephone system can be used to communicate over the telephone from a moving automobile with an opposite party or page the party who is moving in an automobile, the system has gained popularity among busy business persons and is highly evaluated.
The land mobile telephone system comprises an automobile telephone network and an ordinary telephone circuit network which are organically combined. The system is currently operated by a cellular method. In the method, a certain zone is defined as a cell, and these cells are arranged without interstices therebetween. A radio base station is installed in each of the zones so that automobiles running in the zone can constantly communicate by radio with the base station. Radio communication is generally conducted with ca. 800 megahertz, and the base stations are connected by wire with an upstream mobile telephone switching office to form a land mobile telephone network.
FIG. 1 shows an example of land mobile telephone system by the cellular method.
Communication between two mobile terminals 5 or between one mobile terminal 5 and a fixed telephone set connected by wire at an office is generally conducted via radio base stations 2, 3 and 4 and a mobile telephone switching office 1. The office 1 is connected to ordinary telephone lines as well as to radio base stations 2, 3 and 4 by signal lines 50, 51 and 52. These base stations 2, 3 and 4 are each allocated to predetermined service zones 56, 57 and 58 for paging to, answering to and relaying a call of an automobile telephone terminal (hereinafter referred as mobile terminal) in radio which is moving within the zone. The base stations 2, 3 and 4 are provided with antennas 53, 54 and 55 for the purpose. Although the FIG. 1 shows only the representative base stations 2, 3 and 4, other radio base stations are installed respectively in every one of the zones which are arranged to cover the whole space without interstices therebetween, and are wired to the mobile telephone swithing office 1.
FIG. 2 shows the state where the base station 2 further sub-divides a zone for which the station is responsible for service into smaller zones for transmission/receiving of calls.
The antenna 53 of the base station 2 comprises three directional antennas 61, 62 and 63. Each of these antennas 61, 62 and 63 is responsible respectively for transmitting/receiving calls in each of the zones 64, 65 and 66 obtained by further dividing the circumference (360.degree. ) of the station 2 into three (120.degree. ).
The call from a mobile terminal 5 or to a mobile terminal 5 is conducted by paging, answering and speech via the office 1, and radio base stations 2 through 4. The radio base stations 2 through 4 respectively conduct paging and access between mobile terminals by utilizing such control channels as a bi-functional channel which is used for paging as well as for access and a mono-functional channel which is used for paging alone, and after radio connection is made, relay calls between the two telephone terminals by a speech channel designated by the switching office 1.
There is known a land mobile telephone system having two types of control channels. One of the control channels is a mono-functional channel for access dedicated to access such as calling or answering from a mobile terminal and ordering to the mobile terminal, and the other is bi-functional channel for paging and access used commonly for above-mentioned access and paging the mobile terminal. In this system, only the latter channel with the two functions can be used for paging by a radio base station to a mobile terminal, but both of the channels can be used for access between the mobile terminal and the base station. This system therefore is quite advantageous especially when the traffic in the upward direction or to the base station is heavy.
When the function of data receiving of the bi-functional channel of paging and access is disturbed by interference, jamming waves or circuit faults in such a conventional system, the channel cannot receive data, and both functions of receiving and transmitting have to be suspended.
When such a difficulty arises at a radio base station which is provided with the bi-functional channel but without the mono-functional channel, all the services have to be inconveniently suspended in the zone covered by the station. Further, even in the system having the bi-functional channels in a plural number in a service area, the quality of the land mobile telephone service is deteriorated by the degree equivalent to the suspended functions of one bi-functional channel. Moreover, if the bi-functional channels are installed in a larger number, it pushes up a cost.
As stated above, even in the system provided with two types of control channels, when it is broken, only the mono-functional channel can be used for service, incapacitating the function of paging the mobile terminal.
U.S. Pat. No. 4,434,506 discloses a radio communication system where one of the speech channels is switched to be used as a control channel when the control channel is broken. But the system naturally suffers a decrease in the number of speech channels in service.